With ever-increasing data transmission rates, increasing transmission distances, and tighter cost constraints, controlling radiated electromagnetic interference (R-EMI, or simply RE) continues to be a challenge. Because RE adds signals to an already congested radio spectrum, the amount of permissible RE is subject to applicable governmental regulations.
The RE performance of a system may be evaluated by measuring equipment emissions within a narrow frequency reference window at individual frequencies. In the United States, applicable Federal Communications Commission (FCC) regulations dictate using a 120 KHz wide standard reference measurement window, which is swept from about 30 MHz to 1 GHz for purposes of making RE measurement. The International Special Committee on Radio Interference (CISPR) of the International Electrotechnical Commission (IEC) has promulgated similar regulations. Measurement involves a time integration of spectral energy of emissions occurring within the reference measurement window. A measured average emission magnitude at each frequency window is compared to published pre-specified limits, and a determination is made as to whether excessive RE is being radiated. If excessive RE is present, steps are taken to bring the RE-emitting system into compliance within acceptable emission limits.